Air separation plants targeted for production of nitrogen that operate at moderate pressures (i.e. pressures that are higher than conventional air separation unit pressures) have existed for some time. In conventional air separation units, if nitrogen at moderate pressure is desired, the lower pressure column could be operated at a pressure above that of conventional air separation units. However, such operation would typically result in a significant decrease in argon recovery as much of the argon would be lost in the oxygen rich or nitrogen rich streams rather than being passed to the argon column.
To increase the argon recovery in such moderate pressure, nitrogen producing air separation units, a modified air separation cycle was developed in the late 1980s and early 1990s. See, for example, the technical publication Cheung, Moderate Pressure Cryogenic Air Separation Process, Gas Separation & Purification, Vol 5, March 1991 and U.S. Pat. No. 4,822,395 (Cheung). In these prior art documents, a nitrogen and argon producing air separation plant with somewhat high argon recovery is disclosed. The modified air separation cycle involves operating the higher pressure column at a nominal pressure of preferably between about 80 to 150 psia, while the lower pressure column preferably operates at a nominal pressure of about 20 to 45 psia, and the argon column would also preferably operate at a nominal pressure of about 20 to 45 psia. Recovery of high purity nitrogen (i.e. >99.98% purity) at moderate pressure of about 20 to 45 psia is roughly 94%. High argon recovery at 97.3% purity and pressures of between about 20 to 45 psia is generally above 90% but is capped at about 93%.
In the above described prior art moderate pressure air separation cycles, high purity liquid oxygen from the sump of the lower pressure column is used as the refrigerant in the argon condenser rather than kettle liquid. However, when using the high purity liquid oxygen from the sump of the lower pressure column, the argon column needs to operate at higher pressures than conventional argon columns in order to achieve the required temperature difference in the argon condenser. The increase in pressure of the argon column requires the lower pressure column and higher pressure column to also operate at pressures higher than conventional air separation units.
The use of high purity liquid oxygen in the argon condenser also means that the large kettle vapor stream that normally feeds the lower pressure column is avoided, which yields a marked improvement in recovery. As a result, high recoveries of nitrogen, argon, and oxygen are possible with this moderate pressure air separation cycle, even though the elevated pressures would otherwise penalize recovery compared to conventional air separation cycles. The moderate pressure operation of the air separation unit is generally beneficial for nitrogen production, as it means the nitrogen compression is less power intensive and the nitrogen compressor will tend to be less expensive than nitrogen compressors of conventional systems.
Even though the air separation unit in the Cheung publication and U.S. Pat. No. 4,822,395 provides a high purity oxygen vapor exiting the argon condenser, this oxygen stream is not used as oxygen product because the stream exits the process at too low pressure (e.g. 18 psia) and would often require an oxygen compressor to deliver oxygen product to a customer at sufficient pressure. In some regions, use of oxygen compressors are generally unacceptable due to safety and cost considerations. When used, oxygen compressors are very expensive and usually require more complex engineered safety systems, both of which adversely impacts the capital cost and operating costs of the air separation unit.
What is needed is an improved moderate pressure air separation unit and moderate pressure air separation cycle that further improves argon recovery and provides for limited oxygen recovery without the need for oxygen compressors.